Add CMock unit tests for MQTT_ProcessLoop (#963)
Add CMock unit tests for MQTT_ProcessLoop (#963)

* Add setup for testing handleIncomingPublish

* Add coverage for handleIncomingPublish

* Add callbacks for pubrec, pubrel, puback

* Coverage for Acks

* Add coverage for SUBACK and PINGRESP

* Add coverage for all methods except handleKeepAlive

* Add changes to mqtt_utest.c

* Create expectProcessLoopCalls private method to make tests look much cleaner

* Decouple context and expectProcessLoopCalls

* Full coverage

* Add @brief for all methods and variables

* Update expectProcessLoopCalls from adding MQTT_GetPingreqPacketSize

* Update comments

* Address PR comments

* Change parameter names in expectProcessLoopCalls

* Update comments

* Set remaining length when modifying packet type

* Change case of method names

* Address PR comments

* Address PR comments

* Add timer overflow test

* Address PR comments

* Add param docs

* Rename macro to context instead of interface

* Remove modifyIncomingPacket

* Add divider
aws/aws-iot-device-sdk-embedded-C,aws/aws-iot-device-sdk-embedded-C,aws/aws-iot-device-sdk-embedded-C